Remove unused internal 'in-resize' flag

A remnant of days gone by.
Revert "backend: try gdk backend before x11/wayland/egl"

This reverts commit 398a7ac71333208e31d67f3ce50514fab58ba1bb.

We cannot really use the GDK backend without massive regressions inside
the input layer, like touch events and gestures. The GDK backend is not
entirely up to scratch, and it's late in the cycle.

Let's land this early in 3.15, and get it up to par with X11.